Associated with any containment site is the risk of the-contained material (hazardous or non-hazardous) leaking through the barrier, migrating into the surrounding area and possibly contaminating the underlying soil and ground water. Examples of containment sites requiring liquid proof barriers are brine ponds, hazardous material sites, oil storage sites, oil refining sites, and sanitary landfills, or other similar type sites. As environmental regulation becomes more stringent, the old methods of using multiple zones of clay in cellular geometries to establish liquid proof barriers under, around or above containment or other hazardous material sites are being replaced by new methods incorporating polymeric membranes as a means of encapsulation.
One example of a liquid proof barrier is a geomembrane, generally a polymeric material manufactured in long rolls approximately 20 feet in width. Another type of manufactured barrier makes use of a thin bentonite clay layer encapsulated between two layers of a special textile material. Geomembranes and other similar barriers are used both as liquid proof barriers during operation and as part of a cover structure over the containment site at the time of closure. Geomembranes or other similar barriers that have been incorporated into cover structures are required to contain and control biogases emanating from decomposing materials at the site as well as to prevent precipitation and surface water from entering and percolating down through the closed containment site. Hereafter geomembranes will be referred to as membranes and will refer to a resilient flexible material that forms a liquid proof barrier or a containing means for restraining the contents of the containment site. In addition to being flexible it is crucial that membranes be resistant to deterioration as a result of contact with the containment contents or byproducts of the containment contents.
The standard procedure for installing membranes and forming a barrier within the containment site is to lay sheets of the membrane material parallel to one another over the prepared base of the site. The membrane sheets are then fused along their edges, typically with a small crawler welder operating longitudinally at the overlap of the two adjacent membrane sheets-creating double weld zones with a tubular zone between the welds. There is extensive real time visual inspection of these fusion welds and also pressure testing of the tubular zone to see if the welds are leaking. If suspect areas are identified, they are repaired immediately. In addition, the EPA regulations require random sample testing of the weld quality at designated locations along the weld. This testing then requires that a patch be placed and sealed over the test sample hole. Sand, clay and gravel or any combination there of are typically placed over the membrane prior to the introduction of containment site material. Likewise the membrane material is thoroughly inspected at the time of manufacture, as well as, installation for any defects. In most membrane installations, heavy machinery is used to position the membrane and cover the membrane with soil. Membranes used as part of a cover structure are typically installed in a similar fashion, except placed over the containment site, and as such have similar problems to those described above.
Because of the processes used to install membranes within a containment site, even with the weld sampling and testing of the bulk membrane material the integrity of the barrier is largely unknown after installation. As environmental regulations become more stringent, it is critical that there be a reliable and cost effective method for monitoring the long term integrity of the barrier after installation and during the life of the containment site.
Whether the geomembrane is used as a barrier under the site, over the site, or around the site, there is a growing need to be able to ascertain the integrity of the geomembrane or the barrier as well as identify what if any fluids or gases have breached the barrier. There also is a need, but very limited technology currently available, to monitor the long term integrity of a barrier.
A variety of indirect techniques exist for determining membrane integrity. One approach is to use borehole sampling around the site to determine if any contamination plumes exist. The disadvantages are: 1) by the time a plume can be detected a leak would have to be substantial and contamination of soil and ground water would have already occurred and 2) periodic borehole sampling and analysis can be very costly. Another approach is to use electric current in conjunction with a conductive medium under a membrane. U.S. Pat. No. 4,740,757 describes a method by which electrodes are strategically placed under the membrane and electrically coupled through a conductive medium selectively placed between two membranes. U.S. Pat. No. 4,947,470 describes another method in which leaks are detected by placing wires and sensors in a grid pattern in a conductive medium under the membrane. In either of the methods described by U.S. Pat. No. 4,740,757 or U.S. Pat. No. 4,947,470, the disadvantages are similar in that the conductive medium is a mixture of moisture selectively placed under the membrane and the leaking substance. The combination of the conductive medium and a leaking substance can result in a larger contamination plume if the membrane is actually breached. Because of the need for a conductive liquid to be present and the natural presence of condensate caused by soil moisture, there is always a possibility for corrosion of the detectors or wires, impeding the ability to monitor and detect breeches on the barrier or membrane. Additionally, neither of the methods described by the US Patents can qualitatively or quantitatively identify the leaking material.